The nature of the structural alterations in both collagen and hyaluronic acid molecules, as well as the interrelationship between collagen and hyaluronic acid, collagen fibril formation and hyaluronic acid aggregation, will be explored. Photochemical methods along with viscosity, sedimentation, optical rotation and circular dichroism studies will be employed to investigate these areas. These studies will provide an insight to the problem whereby connective tissues undergo aging. In order to determine how hyaluronic acid may modify the properties of collagen, the relationships between UV-induced cross-linking, blue fluorescence and inhibition of fibril formation will be studied both in the absence and presence of hyaluronic acid. The denaturation temperature of UV-irradiated collagen and the thermal profile of blue fluorescences in the presence of hyaluronic acid will also be investigated. Aldehydes of collagen may play a part in UV-induced changes of collagen and such a role will be explored. UV-induced degradation of hyaluronic acid will be studied by determining the action spectra of such systems. The reactions of collagen, hyaluronic acid and collagen-hyaluronic acid mixtures, photosensitized by aromatic amino acids and their photo-products will be explored. This will include studies on the possible photodynamic action of tyrosine-photomodified collagen as a photosensitizer. We also plan to selectively oxidize specific residues in the collagen molecule by using other photosensitizers, binding them either selectively to specific amino acid residues or by carefully controlling the reaction conditions.